1. Field of the Invention
The present invention relates to an A-D converter and an A-D convert method. More particularly, the present invention relates to an A-D converter and an A-D convert method for outputting a digital output signal obtained by digitalizing an analog input signal.
2. Related Art
An A-D converter converts an analog signal into a digital signal. The A-D converter is classified into a single bit mode in which an input signal is quantized by one bit in one clock and a multiple bit mode in which an input signal is quantized by multiple bits in one clock. The A-D converter with a single bit mode includes, for example, a successive-approximation type A-D converter and a delta-sigma type A-D converter. The successive-approximation type A-D converter is disclosed in Ricardo E. Suarez, Paul R. Gray, and David A. Hodges, “An All-MOS Charge-Redistribution A/D Conversion Technique”, IEEE International Solid-State Circuits Conference, 1974, P. 194-195 and 248, James McCreary and Paul R. Gray, “A High-Speed All-MOS Successive-Approximation Weighted Capacitor A/D Conversion Technique”, IEEE International Solid-State Circuits Conference, 1975, P. 38-39 and 211, JAMES L. McCREARY and PAUL R. GRAY, “All-MOS Charge Redistribution Analog-to-Digital Conversion Techniques-Part 1”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. SC-10, No. 6, DECEMBER 1975, P. 371-379. The A-D converter with a multiple bit mode includes, for example, a flash type A-D converter.
Meanwhile, the A/D converter may output an incorrect result in some cases when large noises are instantaneously included in the input signal. In particular, since the successive-approximation type A/D converter sequentially determines values from a high-order bit, the successive-approximation type A/D converter outputs a data value including gross errors when the high-order bit has a wrong comparison result.
In order to solve such a problem, it is considered that the A/D converter includes a low-pass filter having a large capacity in its input stage and removes instantaneously large noises. However, the A/D converter including such the low-pass filter has a delayed settling time, and thus has a long conversion time.